After you put the reindeer antlers on the car, don't open the windows they are attached to while driving, especially if this year you put them on the back windows (unlike last year... see picture of last year to the right). Even if it's a lovely day out and you're driving at a relatively slow speed.
Just be thankful that right one got caught in the window and while the left one landed in the street, you noticed it quickly enough to stop. Also be thankful that the street was not busy at that moment so no one rolled over it and that there was a very kind gentleman who stopped his car, picked up the antler that fell off and ran it toward you as you were running toward it.
Next time, just don't open the windows.